1. Field of the Invention
The present invention concerns a flow-meter, i.e. a device for measuring the instantaneous flow rate of a fluid that flows into a conduit. The flow-meter according to the present invention is particularly, but not exclusively, useful for measuring the oil flow feeding an oil-filled electric cable.
2. DISCUSSION OF THE PRIOR ART
It is known that the measurement of the instantaneous flow of oil for feeding oil-filled cables (hereinafter referred to as O.F. cables) presents several technical problems which flow-meters that are usually employed do not resolve completely and simultaneously. A first problem is related to the wide range within which flow rate can vary. This range is extremely broad, from as low as 0.5 liters per hour to as high as 2,000 liters per hour.
Since known flow-meters do not have the capacity for measurement within such a wide range of values, in practice recourse is had to inserting into the oil feeding line a plurality of flow-meters having diverse capacities and working in parallel with each other; each of these flow-meters covers one limited range of values of the flow rate, so that together the several meters cover the whole required range.
A second problem is related to the desirability of measuring with great precision the instantaneous flow rate at very low values, approaching zero. Such measurement of very low rates of flow is difficult to accomplish because flow-meters generally operate by the displacement of moving parts such as a floating or deformable membrane or the like under the thrust of the fluid, and the mechanical inertia of such moving parts and extremely small displacements resulting from very low rates make such low flow rates difficult to register and to quantify. On the other hand, flow-meters specially designed for measuring very low flow rates are inadequate for measuring much higher flow rates.
Other limitations of conventional flow-meters are their inability to measure flow in both directions, and their inability to transfer the measurement data to a location remote from the point where the measurement is made.